In the processing of digital electrical signals which are transmitted as a series of digital data signals, it is often necessary to monitor the stream of serial incoming data to detect the occurrence of a known data sequence identifying a delimiting character or flag. The delimiting character or flag is a string of digital signals arranged in a predetermined pattern. A flag character, for example, may be a byte consisting of 8 bits. The detection of a flag character has many uses in digital data processing as, for example where the detection of a flag is employed to actuate, interrupt, or modify the operation of associated equipment.
Heretofore the detection of a flag character or flag in a serial data stream has been accomplished by converting the serial data to parallel form and comparing that parallel data with the specified pattern being sought in the data stream. Thus for example, where a flag character of 8 bits is sought, each 8 bit sequence in the serial data stream is converted to parallel form and then compared with the parallel output signals of a register in which the specified pattern is stored. Where the serial data stream is simultaneously monitored for the occurrence of a number of different flag characters, the prior art parallel comparison arrangement may require the employment of additional sets of comparators.